1. Field of the Invention
The present invention relates to a method for producing a reversible thermosensitive recording material which comprises a support and a reversible thermosensitive recording layer formed thereon, which is capable of developing and erasing images repeatedly by utilizing the property of reversibly changing the transparency of the reversible thermosensitive recording layer, depending upon the temperature thereof.
2. Discussion of Background
In recent years, some attention is paid to a reversible thermosensitive recording material capable of temporarily recording images thereon and erasing the same therefrom when such images become unnecessary. As a representative example of that kind of reversible thermosensitive recording material, there is conventionally known a reversible thermosensitive recording material comprising a support and a reversible thermosensitive recording material formed thereon, in which an organic low-molecular-weight material such as a higher fatty acid is dispersed in a matrix resin such as vinyl chloride--vinyl acetate copolymer with a glass transition temperature (Tg) in the range of 50.degree. C. to less than 80.degree. C., as disclosed in Japanese Laid-Open Patent Applications 54-119377 and 55-154198.
When such a conventional reversible thermosensitive recording material is manufactured, an organic solvent such as tetrahydrofuran is employed as a base solvent which dissolves or disperses both the matrix resin and the organic low-molecular-weight material for the reversible thermosensitive recording layer. Organic solvents of this kind have extremely low boiling points and high evaporation rates, so that they soon evaporate from a coating liquid applied to a support for the formation of a reversible thermosensitive recording layer thereon even before the applied coating liquid is dried. As a result, a thin film of the matrix resin is formed on the surface of the recording layer. Due to the formation of the thin film of the matrix resin on the surface of the recording layer, not only the evaporation of the solvent within the recording layer is prevented, but also the particle diameter of the organic low-molecular-weight material dispersed in the matrix resin increases, and the organic low-molecular-weight material separates out on the surface of the recording layer with time.
Moreover, there is the disadvantage that the contact between the support and the thermosensitive recording layer at the interface thereof becomes poor because of the solvent remaining in the thermosensitive recording layer. In addition to the above disadvantage, because of the presence of the residual solvent in the thermosensitive recording layer, there is a problem in that another layer cannot be smoothly overlaid on the surface of the recording layer. In the case where the formation of images and the erasure thereof are repeated many times with the simultaneous application of pressure and heat to the recording material, for instance, by a thermal head, small particles of the low-molecular-weight material gradually grow into large particles, so that the low-molecular-weight material loses the function of scattering light. As a result, the degree of whiteness of the reversible thermosensitive recording layer is eventually decreased and finally the contrast of images is lowered. Furthermore, since the particles of the organic low-molecular-weight material separate out on the surface of the reversible thermosensitive recording layer, even when a protective layer is overlaid on the reversible thermosensitive recording layer, the small particles of the organic low-molecular-weight material in the recording layer migrate to the protective layer and adhere in the form of a dust to a thermal head. The result is that the image formation and the erasure cannot be repeated many times with such a conventional reversible thermosensitive recording material.